What causes the audible hum from some electronics and wires? How is the actual sound being generated?
In: Physics
The electricity supplied to your home switches direction 50 or 60 times a second, usually depending on your country. It’s as if you had a battery as a power supply but you could switch the positive and negative connections back and forth that fast. It’s called “alternating current,” or AC, and the rate is the “mains frequency.” In contrast, the battery alone supplies “direct current,” or DC.
The reason for using alternating current is that a simple magnetic device called a transformer can be used to create higher or lower voltages, depending on how it is wired. You cannot use a transformer to change voltage with DC.
It turns out that if you want to send electricity through wires over long distances, it is more efficient to do so at very high voltages. This is why power lines are generally strung high across tall towers, to keep the wires far from one another and from the ground or towers. On the sending end, the voltage from the generator in a power plant is boosted by a transformer, sometimes up to hundreds of thousands of volts for very long distance runs. On the receiving end, the voltage is lowered using another transformer effectively wired in reverse. The voltage will be lowered even more, in stages along the way, by more transformers until it gets to your home. Search on “electric power distribution” for more on that.
So why the hum? If you’re talking about a hum from an appliance like a microwave, it’s generally due to mechanical vibration from a transformer, because the magnetic field in the transformer is alternating and can be quite strong. The metal core or coils of wire inside it can vibrate or it might be the field interacting with other metal around it.
If it’s from a motor, that’s because most household appliance motors also exploit the AC, and their RPM is a multiple of the mains frequency.
If it’s from an audio device like an amplifier, there can be any number of sources. A common one is the electric and magnetic fields that permeate our homes due to appliances and house wiring. These fields radiate across instruments, cords, cabling and create tiny but annoying AC voltages, which are amplified along with the music.
Sorry this got kind of long, but I hope it helps.
P.S. Spooky thing here. I’m in the US, where the mains frequency is 60 Hz, and when I hear 50 Hz hum, such as in Europe, it sounds kind of growly and scary. I was thinking of posing the question of how “the other” frequency sounds to people and coincidentally found this thread–totally stumbled on it, not as a result of a search.
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